The present invention relates to demodulators which receive a signal transmitted via phase modulation or multi-value QAM modulation, and derive transmitted data from the received signal. The present invention particularly relates to a demodulator used in a receiver of a digital-data-transfer system which transfers data, digital audio signals, digital image signals, or the like.
In the transfer of data, digital audio signals, digital image signals, or the like, symbol data representing binary values or multi-values as digital data is transmitted as carrier-wave modulated signals. The symbol data arrives at a receiver via a transmission line, and demodulator of the receiver decides the symbol data as digital data. The decided data is output from the receiver.
The transmitted signals suffer phase fluctuations as well as magnitude fluctuations because of interference from other communication and/or interference between paths along the transmission line on which a plurality of paths are established. The transmitted signals thus can be distorted before arriving at the receiver. Further, the received signals can further be distorted by noise inside the receiver.
If the distortion of the received signals generated is large, the demodulator cannot ascertain the transmitted symbol data correctly, and ends up outputting incorrect digital data. In order to insure high-quality communication of digital information, the demodulator needs to achieve higher reliability in deciding the symbol data, and needs to decrease data-error rates.
Transmission of digital data is performed by transmitting a signal with a carrier wave thereof modulated in terms of its amplitude or phase in accordance with the digital data to be transmitted. When the transmitted signal is received, however, the actually received signal has distortion introduced along the transmission line.
In consideration of this, a demodulator compensates for the distortion based on the estimation of characteristics of the transmission line, and, then, decides the digital data from the amplitude or phase of the received data. The decided data is supplied as a demodulator output. A well-known method of estimating characteristics of the transmission line includes transmitting predetermined pilot symbols and estimating the characteristics of the transmission line based on the received pilot symbols. This is disclosed in Seiichi Sampei, xe2x80x9cRayleigh Fading Compensation Method for 16 QAM Modem in Digital Land Mobile Radio Systemsxe2x80x9d, Transaction of the Institute of Electronics, Information and Communication Engineers, Vol. J72-B-II, No. 1, pp.7-15 (1988), for example.
The above-described transmission-line-estimation method based on use of pilot symbols transmits at least one of N symbols as a pilot symbol when transmitting the N symbols in one slot. In particular, one pilot symbol is inserted and transmitted as (Nxe2x88x921) data symbols. The receiver then uses the pilot symbol to estimate the characteristics of the transmission line.
A complex envelope of a transmitted pilot symbol is denoted as (3+jxc2x73), and a complex envelope characteristic (i.e., phasing distortion introduced during passage through the transmission line) is denoted as c(k). Further, a received complex base-band signal of the pilot symbol in the receiver is u(k), and a Gaussian white noise is represented as n(k). Then, the following relation is satisfied.
u(k)=c(k)xc2x7(3+jxc2x73)+n(k)xe2x80x83xe2x80x83(1)
On the receiver side, an operation as shown in the following (2) is performed on the received complex base-band signal u(k) by using the transmitted pilot symbol 3+jxc2x73, so as to obtain an estimate of the transmission line c{circumflex over ( )}(k).                                                                                           c                  ^                                ⁡                                  (                  k                  )                                            =                                                u                  ⁡                                      (                    k                    )                                                  /                                  (                                      3                    +                                          j                      ·                      3                                                        )                                                                                                        =                                                c                  ⁡                                      (                    k                    )                                                  +                                                      n                    ⁡                                          (                      k                      )                                                        /                                      (                                          3                      +                                              j                        ·                        3                                                              )                                                                                                          (        2        )            
When the data symbol transmitted from the transmitter is s(k) and the data symbol received at the receiver is r(k), then, the received data symbol r(k) under the effect of the transmission-line characteristic c(k) is represented by the following.
r(k)=c(k)xc2x7s(k)+n(k)xe2x80x83xe2x80x83(3)
An operation as shown in the following (4) is performed on the received data symbol r(k) by using the transmission-line estimate c{circumflex over ( )}(k), so as to obtain the received data symbol r{circumflex over ( )}(k) having the effect of the transmission line removed therefrom.
r(k)=r(k)/c{circumflex over ( )}(k)xe2x80x83xe2x80x83(4)
Here, r{circumflex over ( )}(k) is equal to the transmitted data symbol s(k) if the Gaussian white noise n(k) is zero. In this case, the receiver side can reconstruct the transmitted data symbol s(k) without any effect of the transmission line.
The transmission-line estimate c{circumflex over ( )}(k) becomes identical to the transmission-line characteristic c(k) if there is no Gaussian white noise n(k). Because of the effect of the Gaussian white noise n(k), however, there is a difference between the transmission-line estimate c{circumflex over ( )}(k) and the transmission-line characteristic c(k). In consideration of this, the transmission-line estimate is obtained many times by using a plurality of pilot symbols, and an average is calculated to derive a transmission-line estimate that is highly accurate. Based on the highly accurate transmission-line estimate c{circumflex over ( )}(k), which is obtained by averaging transmission-line estimates corresponding to the plurality of pilot symbols, a received data symbol is decided by removing the phasing deformation of the received data symbol through the operation as shown in the above equation.
As described in the above, a plurality of pilot symbols are required for the averaging operation in order to raise accuracy of the transmission-line estimate c{circumflex over ( )}(k). Namely, p pilot symbols are inserted each time (Nxe2x88x92p) data symbols are transmitted in one slot, and p transmission-line estimates are then averaged. An area where the p pilot symbols occupy is referred to as a pilot block.
An increase in the number p of pilot symbols relative to the number N of transmitted symbols, however, results in a decrease in the number of symbols transmitted as digital data, thereby ending up decreasing a data-transfer rate. Because of this, the number p of the pilot symbols cannot be unconditionally increased.
To cope with this situation, a CDMA demodulating device disclosed in Japanese Patent laid-open Application No. 10-51424 employs a scheme in which a weighted average is calculated with respect to estimates of a channel (transmission-line characteristic) after these estimates are obtained from pilot symbols contained in pilot blocks of a plurality of slots. Another scheme is to obtain an average between a channel estimate obtained from pilot blocks of a plurality of slots and an average of information symbols having polarization thereof corrected.
The method of obtaining a weighted average of transmission-line estimates with respect to pilot blocks of a plurality of slots is accompanied by a processing delay until transmitted symbols of the plurality of slots are received. This is because the estimates of the transmission line are obtained after the pilot blocks of the plurality of slots are received.
There is a need for an accurate estimation of the transmission line without increasing the number p of pilot symbols in one slot and without incurring a processing delay over the plurality of slots. To this end, data symbols placed between pilot symbols may be decided tentatively as digital data, and the transmission-line characteristics may be estimated from the decided data symbols by treating the decided data symbols as pilot symbols. This scheme brings about the same effect as increasing the number of pilot symbols, thereby enhancing an accuracy regarding the estimation of transmission-line characteristics.
In comparison to the transmission-line estimate obtained from known pilot symbols inserted on the transmitter side, when a transmission-line estimate is derived by using tentatively decided data symbols as pilot symbols, this estimate tends to be less reliable to an extend that the tentative ascertainment contains errors. When an error rate of the tentative decided data is large, the transmission-line estimate obtained from the tentatively decided data symbols may be even less reliable then before such data symbols are used.
An object of the present invention is to provide a demodulator which can improve a transmission-line estimate by using tentatively decided data symbols as pilot symbols, and, at the same time, avoids degradation in accuracy of the transmission-line estimate when an error rate of the tentatively decided data symbols grows large.
These and other objects are met by a demodulator according to the present invention which demodulates data symbols by estimating transmission-line characteristics from received pilot symbols. This includes a pilot-based transmission-line-estimation unit which estimate the transmission-line characteristics by using the pilot symbols. A tentative-data-decision unit which tentatively decides the data symbols based on a transmission-line estimate that is output from the pilot-based transmission-line-estimation unit. A data-based transmission-line-estimation unit which estimates the transmission-line characteristics by using tentatively decided data that is output from the tentative-data-ascertainment unit. Multiplying units which multiply the transmission-line estimate output from the pilot-based transmission-line-estimation unit and a transmission-line estimate output from the data-based transmission-line-estimation unit by respective weights. A ratio between the weights varying in accordance with reliability of the tentatively decided data. A pilotanddata-based transmission-line estimation unit which obtains a transmission-line estimate by averaging weighted transmission-line estimates output from the multiplying units. A final-data-decision unit which finally decides the data symbols by using the transmission-line estimate output from the pilotanddata-based transmission-line-estimation unit.
The demodulator is characterized in that the tentative-data-ascertainment unit outputs a signal indicative of reliability of the tentatively decided data based on a result of soft decision of the received data symbols. The multiplying unit which multiplies the transmission-line estimate output from the data-based transmission-line estimation unit by one of the respective weights changes the one of the respective weights in accordance with the signal indicative of reliability of the tentatively decided data.
The demodulator is characterized in that the signal indicative of reliability of the tentatively ascertained data represents at least two different reliability levels, and the multiplying unit which multiplies the transmission-line estimate output from the data-based transmission-line estimation unit by the one of the respective weights makes step changes to the one of the respective weights in accordance with the signal representing the at least two different reliability levels.
The demodulator is characterized by further comprising a transmission-mode-check unit which checks a transmission mode of digital-data-transmission signals, wherein the transmission-mode-check unit outputs a signal indicative of reliability of the tentative-data-ascertained data where the reliability varies depending on the transmission mode. The multiplying units use the respective weights having the ratio therebetween varying depending on the signal indicative of reliability of the tentatively decided data supplied from the transmission-mode-check unit.
The demodulator is characterized by receiving and demodulating signals directed to other receivers or transmitted via other channels when the signals are transmitted from a single transmission source. The pilot-based transmission-line estimation unit, the data-based transmission-line estimation unit, and the multiplying units are provided with respect to each of a signal directed to a receiver of the demodulator and the signals directed to other receivers or transmitted via other channels. The pilotanddata-based transmission-line estimation unit obtains a weighted average of the pilot-based transmission-line estimate in accordance with the reliability of the tentatively decided data with respect to all signals including the signal directed to the receiver of the demodulator and the signals directed to other receivers or transmitted via other code channels, thereby obtaining a transmission-line estimate.
A demodulator which demodulates data symbols by estimating transmission-line characteristics from received pilot symbols, includes a pilot-based transmission-line estimation unit which estimate the transmission-line characteristics by using the pilot symbols. A first tentative-data-decision unit which tentatively decides the data symbols based on a transmission-line estimate that is output from the pilot-based transmission-line estimation unit. A first data-based transmission-line estimation unit which estimates the transmission-line characteristics by using tentatively ascertained data that is output from the first tentative-data-ascertainment unit. First multiplying units which multiply the transmission-line estimate output from the pilot-based transmission-line estimation unit and a transmission-line estimate output from the first data-based transmission-line estimation unit by respective weights, which are varied in accordance with reliability of the tentatively ascertainment data. A first pilotanddata-based transmission-line estimation unit which obtains a transmission-line estimate by averaging weighted transmission-line estimates output from the first multiplying units. A second tentative-data-decision unit which tentatively decides the data symbols again based on the transmission-line estimate that is output from the first pilotanddata-based transmission-line estimation unit. A second data-based transmission-line estimation unit which estimates the transmission-line characteristics by using tentatively decided data that is output from the second tentative-data-decision unit.
Second multiplying units which multiply the transmission-line estimate output from the pilot-based transmission-line estimation unit, the transmission-line estimate output from the first data-based transmission-line estimation unit, and a transmission-line estimate output from the second data-based transmission-line estimation unit by respective weights, which are varying in accordance with reliability of the tentatively ascertained data. A second pilotanddata-based transmission-line estimation unit which obtains a transmission-line estimate by averaging weighted transmission-line estimates output from the second multiplying units, and a final-data-decision unit which finally decides the data symbols by using the transmission-line estimate output from the second pilotanddata-based transmission-line estimation unit.
The demodulator is characterized in that the first and second tentative-data-ascertainment units, the first and second data-based transmission-line estimation units, the first and second multiplying units, and the first and second pilotanddata-based transmission-line estimation units perform tentative data decisions, estimation of the transmission-line characteristics based on the tentatively decided data. Multiplication of the respective weights, which are varied in accordance with reliability of the tentatively decided data or the respective weights varying in accordance with reliability of the tentatively decided data and depending on which stage is current. Estimation of the transmission-line characteristics based on the pilot and tentatively decided data, respectively, repeatedly over many stages.